In order to enhance performances of various safety devices, for example, airbags and seatbelts mounted to a vehicle, operations of the safety devices are controlled in accordance with a weight of an occupant that is seated in a seat according to known apparatuses. For example, when the occupant is seated in the seat without wearing a seatbelt, an alarm indicates that the seatbelt is unbuckled. Further, the law stipulates that a passenger-side airbag is to be deployed at a vehicle collision when an adult is seated in the passenger seat. The law further stipulates that a deployment of an airbag should be prohibited in a case where a child safety seat is fixed to the passenger seat facing backward so that an infant or a child and a driver can see each other because an impact by the deployment of the airbag causes an adverse effects. Under the law, a weight of a smaller female adult is applied as a criterion for determining whether an occupant is an adult. The law also stipulates a criterion for determining whether an occupant is an infant or child. Thus, detecting a weight of an occupant accurately to obtain a correct determination of an occupancy state, or types of occupant is important to ensure a safety of the occupant.
A known load detecting structure for a vehicle seat, which measures a weight of an occupant, that is, a load applied to support portions of a seat, is disclosed in JP3904913B (Patent reference 1). According to the known load detecting structure for the vehicle seat, a load detection means which includes a strain gauge is provided between each of seat-side lower rails serving as the support portion of the seat and each of floor-side leg members so that the loads detected by the four load detection means are added to obtain the weight of the occupant. Known apparatuses which detect a portion of the load by a load detection means which is provided at a part of support portions are adopted because the accurate load is not necessarily measured for determining whether or not a passenger occupies a seat or whether an occupant is an adult or an infant/child. According to a known seat weight measuring apparatus disclosed in JP2001-150997A (Patent reference 2), a load sensor is provided only at a rear-center support portion among three support portions including a fore-left support portion, a fore-right support portion, and the rear-center support portion. Accordingly, the number of load sensors is reduced to reduce costs of parts and costs for assembling and wiring.
With constructions of apparatuses such as Patent reference 2 for detecting a part of load by a load detection means provided at a part of support portions, a distribution ratio of the load applied to the part of the support portions changes in response to changes of postures of the occupant and changes in positions of the occupant, and a detected load value varies. In order to reduce the influence of the changes of postures of the occupant and the changes in positions of the occupant to the load value to be detected, Patent reference 2 discloses a positional sensor which detects a position of the occupant on the seat.
The known apparatus disclosed in Patent reference 2, which detects a part of the load applied to the support portions is favorable in terms of enabling to reduce the cost. However, according to the construction of the apparatus disclosed in Patent reference 2, a precision of the determination of types of occupant declines compared to the apparatus disclosed in Patent reference 1, which detects all of the loads applied to the support portions. For example, with the load sensor provided at the rear portion of the seat, a detected load value declines when an occupant is seated at a front portion of the seat or when an occupant is seated in the seat leaning forward without leaning against a seatback. In those circumstances, even if the occupant is an adult, the apparatus may determine that the occupant is an infant/child. In a case where the additional positional sensor is provided similar to Patent reference 2 in order to enhance the precision of the determination of types of occupants, the manufacturing cost increases with each positional sensor, thus the advantages of reducing the load sensor are ineffective.
Further, an angle of inclination of the vehicle fluctuates the detected load value applied to the support portions. For example, when the vehicle travels downhill, the detected load value is assumed to be smaller compared to when the vehicle travels on a flat road because the load sensor provided at the rear portion of the seat receives less ratio of the weight of the load applying body seated on the seat when the vehicle travels the downhill than when the vehicle travels on the flat road. In a case where the aforementioned causes such as positions and postures of the occupant are added, accurate determination of types of the occupant becomes further difficult.
A need thus exists for a seat load determining apparatus which is not susceptible to the drawback mentioned above.